1. Field of the Invention
This invention relates to providing synchronization distribution throughout a network and in particular relates to providing this distribution in a synchronous optical communications environment prevalent in telephone networks.
2. Description of the Environment
In digital telephone networks, the network is comprised of hundreds or even thousands of offices or nodes such as shown in simplified form in FIG. 1. The network 10 has a plurality of offices 12, 14, 16, 18, 20. Each node has a local timing source 12a, 14a, 16a, 18a, 20a, commonly called a BITS for Building Integrated Timing Supply. Also, each node has a variety of equipment such as switches, optical multiplexers, channel blanks, etc. commonly referred to as network elements (NE's) 12b, 14b, 16b, 18b, 20b, with the timing for each network element within the office being supplied by the office's BITS. The various offices within the network are connected by copper or optical fiber links 22 called facilities. Unlike the earlier versions of the copper based networks where the facilities formed a mesh type network with each office being linked to multiple offices by several facilities, digital optical networks are arranged in chains or rings with facilities tying each office typically to the two adjacent offices.
Further, in a typical digital network, there are a plurality of primary reference source checks called PRS checks. Typically, the PRS clocks are implemented using cesium beam or GPS receiver technology. The PRS clocks serve as master clocks and provide a timing reference for the remainder of the network. The PRS timing is communicated over the facilities to different nodes to permit synchronization between various nodes within the network.
The earlier (non-standard versions of the optical fiber network employed asynchronous bit stuffing techniques to multiplex the input tributary signals onto the optical line. The distribution of the timing reference in such a network may be realized using a embedded DS1 signal, as shown in FIG. 2. The PRS timing 30 in an office 32 is provided to the BITS 35 and then to fiber multiplexer 36 in a first office and communicated to the next office 40 over the embedded DS1 signal in the optical facility 38. Further, the fiber multiplexer 42 at the next office 40 recovers the DS1 clock 44 and passes that recovered clock to the BITS 46 of the second office and to the fiber multiplexer 48 for transmission over the next facility in the chain to the next office. Since the BITS clock 46 is not used for generating the line timing signal 50 provided to the next office in the chain (not shown), inaccuracies in the timing reference communicated to the BITS timing in intervening offices do not effect the timing reference communicated to the BITS of the successive offices. Thus, if the BITS timing reference in the second office malfunctions, the synchronization of the successive nodes or offices (not shown) in the network is unaffected. Therefore, each of the nodes or offices in the network may be thought of receiving its synchronization timing directly from the offices containing the PRS. Where each of the nodes of the network is receiving the timing reference directly from the PRS, synchronization may be thought of as being at the same level. Such distribution schemes of synchronization are referred to as being flat.
Although the method described above yields the desirable flat synchronization distribution system, it is not deployed extensively in the telephone network for two reasons. First, the bit stuffing operation performed at each node adds jitter to the embedded DS1 synchronization reference. This may render the DS1 signal unusable as a timing reference after it traverses a few nodes. Second, and more important, the nonstandard asynchronous optical fiber systems are being replaced by the recently developed standard synchronous optical network technologies, designated as SONET or SDH. The method of distributing the synchronization reference using an embedded DS1 signal does not work properly in the SONET environment, as explained below.
In a SONET multiplexer, the output optical line clock is normally synchronized to the office BITS clock. The rate variations between the input tributaries and the output line signal are accommodated by a byte stuffing process known as pointer adjustment. The eight bit phase movements caused by the pointer adjustments can be large enough to render the embedded DS1 timing reference incapable of adequately transporting the synchronization information. Hence, the standards organizations (ANSI and the ITU) recommend that a DS1 signal embedded within a SONET line signal not be used for synchronization distribution. Instead they recommend the use of the recovered optical line clock to generate a derived DS1 synchronization signal. This derived DS1 signal serves as the synchronization reference input to the office BITS clock.
The use of the derived DS1 to distribute synchronization references, however, implies a hierarchical synchronization network. In such a network, the BITS clock at an intermediate node is not synchronized directly to the PRS but is instead synchronized to the timing reference supplied by the BITS clock in the previous node. This hierarchical scheme for the distribution of synchronization signals has many shortcomings.
First, administrative controls are required to ensure that a higher quality BITS clock (lower stratum number) does not accept timing from a lower quality BITS clock. Second, the cascade of clocks created by the hierarchical chain can impair the timing reference traversing the network. Third, if a BITS clock fails anywhere in the chain, all the downstream clocks will lose synchronization. And finally, this scheme is prone to the inadvertent creation of timing loops, especially under facility failure conditions. (A timing loop occurs when timing from a first node is passed to the second node and then the timing is fed back through a chain of one or more additional nodes to the first node so that the first node is synchronizing its timing to itself. Such a situation is clearly undesirable since all the nodes involved in the timing loop will be isolated from the PRS).
Synchronization messaging is a solution recommended by the standards organizations to alleviate some of the shortcomings delineated above. In this method, the status of the clock that generates the timing reference at a particular node is communicated to the clocks and network elements at other nodes over a messaging channel. The clocks at these other nodes will then decide, in an intelligent manner, whether they should synchronize to one of the incoming timing references, or whether they should operate in a holdover mode. However, the synchronization messaging scheme does not cure all the problems created by the hierarchical synchronization distribution network. Furthermore, implementation of this messaging scheme will be expensive as it involves the retrofitting of the existing BITS clocks and the SONET network elements to provide this capability.
Therefore, it is the first objective of this invention to provide a method for transporting network synchronization reference signals over the existing SONET network using a flat distribution scheme. It is a second objective of this invention to distribute these synchronization reference signals without incurring the problems associated with the hierarchical scheme. It is yet a third objective of this invention to achieve the flat synchronization distribution system without requiring substantial hardware investment or retrofitting costs.